SELECTION OF TIME-HISTORIES FOR BRIDGE DESIGN IN EUROCODE 8 - PDF by fad10689

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									       Proc. of 1st US-Italy Seismic Bridge Workshop, EUCENTRE, Pavia, Italy 18 - 20 April 2007.




SELECTION OF TIME-HISTORIES FOR BRIDGE DESIGN
IN EUROCODE 8


IUNIO IERVOLINO, GIUSEPPE MADDALONI, EDOARDO COSENZA AND
GAETANO MANFREDI
Dipartimento di Ingegneria Strutturale, Università degli Studi di Napoli Federico II


Abstract:       Eurocode 8 (EC8) allows the use of real earthquake records as an input for
                time-history analysis of structures. In its Part 2, the code discusses the
                preparation of seismic input for bridge structures; although referring to the
                same design spectral shapes of Part 1, the prescriptions for bridges are
                somewhat differently specified in respect to those for buildings. The main
                requirement the chosen record set should satisfy, is the compatibility of the
                horizontal average SRSS spectrum with the design spectrum in a broad range
                of periods. The set has to be made of at least three recordings, but seven is
                the minimum set size to consider the mean structural response as the design
                value. The code seems to indicate real records as the principal source of
                ground motions the practitioner should rely on, however the selection of
                real records is not straightforward and requires searching of large databases
                to find sets compliant with the code spectrum. In another study, the authors
                discussed the record selection prescriptions in EC8 Part 1, herein these
                issues are reviewed in respect to the specific requirements given in EC8 Part
                2, and the European Strong-motion Database is searched to identify real
                record sets matching the design spectral shapes for several site conditions.

Key words: Time-history, Eurocode 8, Records, Bridges.

1. INTRODUCTION
The availability of on-line, user-friendly, databases of strong-motion recordings and the
rapid development of digital seismic networks worldwide have increased the accessibility
to natural accelerograms. This is the main reason why they currently seem to be the most
attractive option to define the seismic input in order to assess seismic structural
2            Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



performance. Therefore, earthquake engineering research has focused lately on the
selection of real ground motions for non-linear structural analysis, and relatively simple
procedures have been developed to link records to the hazard at the site [Cornell, 2004].
In spite this effort, in many seismic codes the guidelines about preparation of ground
motion input for dynamic analysis are poor, especially in those cases when bi-directional
loading is required [Beyer and Bommer, 2007]. This is partially because research on the
topic is developing fast and at least a few years are required by regulations to take it in.
Norms are similar worldwide, the minimum set size is typically 3 to 7 records, and the
main prescription is the compatibility with the design spectrum in a specified range of
periods. Eurocode 8 (EC8) Part 1 [CEN, 2003] asks that the average spectral ordinates of
the chosen set should match the design spectrum with some tolerance. Little, if any,
prescriptions are given about other features of the signal. EC8 Part 2 [CEN, 2005],
referring to bridges, has similar prescriptions but more detailed; it refers to the same
spectral shapes and site conditions given in Part 1 and still requires three-dimensional
seismic input, but it specifies that the matching with the design spectrum should be
carried out considering the square root summation of squares (SRSS) of the two
horizontal spectra independently of the vertical component. Other particular conditions
about near source and spatial variability of seismic motion are also specifically considered
in Part 2. EC8, in preparation of seismic input, requires consistency of the chosen record
set with the specific features of the seismic sources the site of interest is subjected to,
however the spectral shapes given are related to the hazard only via the anchoring value
(ag). EC8 does not provide anchor values for its non-dimensional spectral shapes. The ag
values considered herein correspond to the Italian case, where the seismic territory is
divided into four zones representing different hazard levels, where seismic resistant
design is mandatory only in the upper three zones1.

In another study [Iervolino et al., 2007] the authors discuss the EC8 Part 1 requirements
for real accelerograms in respect to the best current practice in record selection. In the
same study, the European Strong-motion Database (ESD) is investigated to find real
record sets complying as much as possible with EC8 spectra. Herein the EC8 Part 2
prescriptions are considered. The ESD dataset has been investigated in order to find un-
scaled (original) record sets respecting as much as possible the spectral matching
requirements. Moreover, sets of scaled code-compatible accelerograms were also
considered in order to reduce the record-to-record variability of the spectra, and to obtain
sets which are independent of the anchoring value of the code spectrum. Finally, sets are
found and some examples are shown herein.


1 The ag values for the Zone 1, 2 and 3 are 0.15g, 0.25g and 0.35g respectively. In fact, the if the peak ground acceleration

(on rock) with a 10% exceeding probability in 50 years falls in one of the intervals ]0.25g, 0.35g], ]0.15g, 0.25g], or ]0.05g,
0.15g], then the site is classified as Zone 1, 2 or 3 respectively [OPCM 3519, 2006].
                                           1st US-Italy Seismic Bridge Workshop                                   3


2. SEISMIC INPUT FOR NON-LINEAR DYNAMIC ANALYSIS IN EUROCODE 8 PART 2
In EC8 the seismic input for time-history analysis is defined after the elastic response
spectrum. In Part 1 the spectral shapes are given for both horizontal and vertical
components of motion. Section 3.2.22 gives two spectral shapes defined as type 1 and type 2.
The latter applies if the earthquake contributing most to the seismic hazard has surface
waves magnitude not grater that 5.5, otherwise the former should be used. The design
spectral shape for the vertical component has the same distinction. In Figure 1 the 5%
damped elastic spectra for the five main site classes are given as normalized in respect to
ag. The anchoring value for the vertical elastic spectrum (avg) is defined by the suggested
ratio avg/ag=0.9. EC8 Part 2 refers to the same spectral shapes in order to define the
seismic input for time-history analysis of bridges.




                             Figure 1. EC8 horizontal and vertical type 1 spectral shapes



2.1        General requirements and spectral matching
The requirements for the seismic input for dynamic analysis are given in section 3.2.3: when
a non-linear time-history analysis is carried-out, at least three pairs of horizontal ground motion time-
history components shall be used. The pairs should be selected from recorded events with magnitudes,
source distances, and mechanisms consistent with those that define the design seismic action. When the
required number of pairs of appropriate recorded ground motions is not available, appropriate modified
recordings or simulated accelerograms may replace the missing recorded motions.

The chosen set of accelerograms, should match the following criteria:




2   In the rest of the paper all calls and verbatim citations of Eurocode 8 will be simply indicated in italic.
4           Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



           a) for each earthquake consisting of a pair of horizontal motions, the SRSS spectrum shall be
           established by taking the square root of the sum of squares of the 5%-damped spectra of each
           component;

           b) the spectrum of the ensemble of earthquakes shall be formed by taking the average value of
           the SRSS spectra of the individual earthquakes of the previous step.

           c) The ensemble spectrum shall be scaled so that it is not lower than 1,3 times the 5% damped
           elastic response spectrum of the design seismic action3, in the period range between 0,2T1 and
           1,5T1, where T1 is the natural period of the fundamental mode of the structure in the case of a
           ductile bridge, or the effective period (Teff) of the isolation system in the case of a bridge with
           seismic isolation.

           d) When the SRSS spectrum of the components of a recorded accelerogram gives accelerations the
           ratio of which to the corresponding values of the elastic response spectrum of the design seismic
           action shows large variation in the period range in (c), modification of the recorded accelerogram
           may be carried out, so that the SRSS spectrum of the modified components is in closer
           agreement with the elastic response spectrum of the design seismic action.

           e) When three component ground motion time-history recordings are used for nonlinear time-
           history analysis, scaling of the horizontal pairs of components may be carried out in accordance
           with previous step, independently from the scaling of the vertical components. The latter shall be
           effected so that the average of the relevant spectra of the ensemble is not lower by more than 10%
           of the 5% damped elastic response spectrum of the vertical design seismic action in the period
           range between 0,2Tv and 1,5Tv, where Tv is the period of the lowest mode where the response to
           the vertical component prevails over the response to the horizontal components (e.g. in terms of
           participating mass).

           f) The use of pairs of horizontal ground motion recordings in combination with vertical
           recordings of different seismic motions, is also allowed. The independent scaling of the pairs of
           horizontal recordings and of the vertical recordings shall be carried out.

In section 4.2.4.3, the code allows the consideration of the mean effects on the structure,
rather then the maximum, when non-linear dynamic analysis is performed for at least
seven independent ground motions.




3 EC8 Part 1 specifies that the chosen set should not underestimate the code spectrum more than 10%. The prescription of
EC8 Part 2 seems to be equivalent because the SRSS, for equal spectra in the two directions, is exactely one of the two
spectra times 1.4.
                                  1st US-Italy Seismic Bridge Workshop                                      5


2.2 Vertical component of seismic action and other issues in record selection
As specified in section 4.1.7, the effects of the vertical seismic component on the piers may
be omitted in cases of low and moderate seismicity, while in zones of high seismicity
these effects need only be taken into account in the following conditions: (a) if the piers are
subjected to high bending stresses due to vertical permanent actions of the deck; (b) when the bridge is
located within 5 km of an active seismotectonic fault; (c) for a prestressed concrete deck; (d) the effects of
the vertical seismic component on bearings and links shall always be taken into account.

EC8 Part 2 has specific prescriptions for special cases as near-source conditions. In
particular, the code prescribes that site-specific spectra considering near source effects
shall be used, when the site is located horizontally within 10 km of a known active
seismotectonic fault that may produce an event of Moment Magnitude higher than 6.5.
The code also prescribes when the spatial variability of ground motion has to be
considered (section 3.3): when the soil properties along the bridge vary to the extent that
more than one ground types correspond to the supports of the bridge deck; or when soil
properties along the bridge are approximately uniform, but the length of the continuous
deck exceeds the distance beyond which the ground motions may be considered
uncorrelated. These special cases were not considered in the present study.

3. SELECTION CRITERIA CONSIDERED
Herein it was not possible to select natural records from events having source features
consistent with those that define the design seismic action as prescribed in section 2,
because this study does not refer to any site-specific case. However, it has to be noted
that the design spectrum is related to the seismic hazard at the site only through the ag
value, which is often provided by the national authorities and it is not common for
engineers involved in design to have source parameters to match. Other spectral
matching requirements given in section 2 were considered along with additional criteria,
which allowed to rank the record sets found. The additional criteria considered are:

a) the deviation of the average spectrum in respect to the code spectrum (δ), which gives
a measure of how much the mean spectrum of a records’ combination deviates from the
spectrum of the code. Definition of δ is given in Eq. (1), where: Sao,med (Ti ) is the ordinate
of the average spectrum corresponding to the period Ti; Sas (Ti ) is the value of 1.3 [1.0]
times the ordinate of the code spectrum at the same period horizontal [vertical] spectrum;
and N is the number of values within the considered range of periods. Selecting a record
set with a low δ may allow to obtain an average spectrum which is well approximating the
code. The deviation δ was computed separately for the horizontal SRSS and the vertical
components of motion, and indicated as δH and δV respectively.
6         Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi




                                                        ( T ) − Sa ( T ) ⎞
                                                                             2
                                         1   N    ⎛ Sa
                                 δ=
                                         N
                                             ∑ ⎜ o ,med Sai (T ) s i ⎟
                                                  ⎜
                                             i =1 ⎝
                                                                         ⎟
                                                                                            (1)
                                                           s   i         ⎠

b) The maximum deviation of a single spectrum within a set in respect to the code
spectrum (δmax), which is defined as in Eq. (1) replacing Sao,med (Ti ) with the ordinates of a
single spectrum of the combination. Controlling this parameter may allow choosing
combinations characterized by records having the individual spectra relatively close to the
reference spectrum, and therefore being narrowly distributed around it.
c) The number of different events within a set. This criterion corresponds to the
identification of combinations of records featuring the largest number of different events
possible.


4. WAVEFORM DATABASE AND ANALYSES
The investigated database is the European Strong-motion Database, whose URL is
http://www.isesd.cv.ic.ac.uk; see Ambraseys et al. [2000, 2004] for further information.
Selecting the recording stations by the site class (rock, stiff soil, soft soil, very soft soil,
and alluvium) allows to download all the spectra of accelerograms recorded on each
ground category. The website was accessed in April 2005 (April 2007 for A site class).
Stations without all three translational components were discarded. Moreover, only
events characterized by a Moment Magnitude equal or larger than 5.8 (6.0 for site class A)
have been retained. For D site class, no reduction to the initial record list was made
because of the shortage of stations for that geological condition. The resulting numbers
of horizontal records are given in Table 1, where the two horizontal and vertical
components are conventionally indicated as X, Y and Z respectively.

                          Table 1. Input dataset considered in the study
                    Local Geology                X            Y                  Z
                    A (rock)                     134          134                134
                    B (stiff soil)               134          134                134
                    C (soft soil)                122          122                122
                    D (very soft soil)           28           28                 28
                    E (alluvium)                 29           29                 29


The identified database of records was investigated, via a specifically developed computer
code, in order to find sets that consist of 7 earthquakes (sets made of 7 pairs considering
the SRSS of the 5% damped spectra of both horizontal components, and the 7 vertical
components), for all five site classes. The code analyzed all possible combinations of
                                      1st US-Italy Seismic Bridge Workshop                                               7


spectra of the input list checking the match with the code shapes. The compatibility
interval was chosen to be 0.139 sec - 4 sec for horizontal and 0.04 sec – 2 sec for vertical
components. These intervals, according to condition (c) and (e) of section 2, render the
record sets found suitable for structures with horizontal fundamental period T1 in the
range 0.7 sec – 2.7 sec and for structures with vertical fundamental period Tv in the range
0.2 sec – 1.3 sec. For each combination, the software also computes the deviation of each
single spectrum within the set, and also the deviation of the average spectrum from the
code spectrum. Results of this search were manually 4 ranked with respect to the
additional criteria given in the previous section. In the following the total number of sets
compatible with EC8 spectra is presented and selected results, referring to both un-scaled
(original) and normalized (non-dimensional) records are displayed. It has to be anticipated
that for D and E soil types, no results were found. This is primarily due to the shortage of
earthquake recordings on these soils in the database.


4.1 Sets of un-scaled (original) spectra
The chosen dataset has been investigated for sets made of 7 un-scaled SRSS horizontal
spectra. The average has been calculated on seven spectra, obtained from 14 original
records, by taking the square root of the sum of squares of both X and Y components of
seven recording stations. A summary of results is given in Table 2, which shows, for any
site class and seismic zone for which combinations exist, the number of spectrum-
compatible sets found and the corresponding tolerance in matching the average spectrum
imposed in the analyses (it is to recall here that the compatibility lower bound assigned by
EC8 is 30% above the code spectrum). The compatibility upper bound was adjusted
adaptively find more Eurocode spectra compatible sets. As it is expected, the larger
number of results corresponds to the lower hazard levels. It should also be noted that for
the higher hazard levels of soil C, it was not possible to find results satisfying the EC8
prescriptions5. In fact, the lower bound had to be relaxed; sets found in this case may be
linearly scaled to comply with the code spectrum.

For those combinations found and listed in Table 2, it has been verified whether the sets
made of vertical components match the code spectrum, without scaling. Results of this
analysis are given in Table 3. For A and C ground types, it was not possible to find results
satisfying the spectral matching requirements. For this reason, the lower bound was
relaxed; again, sets found may be linearly scaled to comply with the code spectrum.


4Another more refined option to carry out this job via genetic algorithms is that proposed by Naeim et al. [2004].
5 The compatibility lower bounds for Zone 1 of C site class are given in Table 2; 0% above the code spectrum is the
minimum tolerance to find results. These levels have been obtained iteratively reducing (with a 10% step) the lower bound
in the analyses. In other words it was not possible to find suitable results using 30%, 20% and 10% respectively. The upper
bounds have been chosen arbitrarily to obtain a significant number of resulting record sets.
8              Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



                                  Table 2. Compliant horizontal SRSS sets found
        Ground       Zone    Compatibility Lower Bound6           Compatibility Upper Bound6   Sets Found
        A            1       30%                                  1000%                        4
                     2       30%                                  100%                         127177
                     3       30%                                  70%                          380385
        B            1       30%                                  1000%                        53
                     2       30%                                  80%                          4179
                     3       30%                                  80%                          838263
        C            1       0%                                   1000%                        10
                     2       30%                                  100%                         167
                     3       30%                                  80%                          115307
                                     Table 3. Vertical components’ sets found
        Ground        Zone   Compatibility Lower Bound7           Compatibility Upper Bound6   Sets Found
        A             1      25%                                  1000%                        3
                      2      30%                                  1000%                        18
                      3      40%                                  1000%                        226
        B             1      10%                                  1000%                        7
                      2      10%                                  1000%                        41
                      3      10%                                  1000%                        8207
        C             1      55%                                  1000%                        3
                      2      30%                                  1000%                        1
                      3      30%                                  1000%                        99



As an example, selected results are given from Figure 2 to Figure 10. They correspond to
all three hazard levels of all site classes. In the figures the rough thick curve represents the
average spectrum and the thick smooth curve represents the code spectrum. The dashed
line is the compatibility limit prescribed by the code; thin lines are the individual spectra
within a set. In the legend of any figure the seven digits station code, as well as the
earthquake code (EQ) from the ESD database, is given. The legend, when needed, also
reports the value of scale factors to comply with EC8 spectra. For details about the
records displayed see the Appendix.




6   Expressed in terms of overestimation of the code spectrum.
7   Expressed in terms of underestimation of the code spectrum.
                                               1st US-Italy Seismic Bridge Workshop                                                  9


                                                                        Horizontal SRSS

                                                                                                               000055 EQ:34
                             1.5                                                                               000182 EQ:87
                                                                                                               000198 EQ:93
                                                                                                               000290 EQ:146
                                                                                                               006332 EQ:2142
                    Sa [g]




                              1                                                                                006349 EQ:2142
                                                                                                               007142 EQ:2309

                             0.5



                              0
                                   0     0.5          1         1.5            2            2.5          3         3.5           4
                                                                          Period [sec]

                                                                            Vertical

                                                                                                        000055 EQ:34 SF:1.65
                             1.5                                                                        000182 EQ:87 SF:1.65
                                                                                                        000198 EQ:93 SF:1.65
                                                                                                        000290 EQ:146 SF:1.65
                                                                                                        006332 EQ:2142 SF:1.65
                    Sa [g]




                              1                                                                         006349 EQ:2142 SF:1.65
                                                                                                        007142 EQ:2309 SF:1.65

                             0.5



                              0
                                   0   0.2      0.4       0.6     0.8          1          1.2     1.4        1.6     1.8         2
                                                                          Period [sec]


Figure 2. Site class A – Zone 1. Set with minimum average deviation from the horizontal target spectrum (δH = 0.235).
                                           SF is the individual scale factor.

                                                                        Horizontal SRSS

                                                                                                               000055 EQ:34
                             1.5                                                                               000182 EQ:87
                                                                                                               000198 EQ:93
                                                                                                               000200 EQ:93
                                                                                                               001228 EQ:472
                    Sa [g]




                              1                                                                                004674 EQ:1635
                                                                                                               007142 EQ:2309

                             0.5



                              0
                                   0     0.5          1         1.5            2            2.5          3         3.5           4
                                                                          Period [sec]

                                                                            Vertical

                                                                                                        000055 EQ:34 SF:1.35
                             1.5                                                                        000182 EQ:87 SF:1.35
                                                                                                        000198 EQ:93 SF:1.35
                                                                                                        000200 EQ:93 SF:1.35
                                                                                                        001228 EQ:472 SF:1.35
                    Sa [g]




                              1                                                                         004674 EQ:1635 SF:1.35
                                                                                                        007142 EQ:2309 SF:1.35

                             0.5



                              0
                                   0   0.2      0.4       0.6     0.8          1          1.2     1.4        1.6     1.8         2
                                                                          Period [sec]




   Figure 3. Site class A – Zone 2. Set with minimum single-record deviation from the horizontal and vertical target
spectrum (δHmax = 0.808; δVmax = 0.513) and minimum average deviation from the vertical target spectrum (δV = 0.231).
                                            SF is the individual scale factor.
10            Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



                                                                         Horizontal SRSS

                                                                                                               000052 EQ:34
                               1.5                                                                             000182 EQ:87
                                                                                                               000198 EQ:93
                                                                                                               000200 EQ:93
                                                                                                               001255 EQ:472
                      Sa [g]




                                1                                                                              004679 EQ:1635
                                                                                                               006335 EQ:2142

                               0.5



                                0
                                     0     0.5         1         1.5            2            2.5         3         3.5           4
                                                                           Period [sec]

                                                                             Vertical

                                                                                                         000052 EQ:34 SF:1.5
                               1.5                                                                       000182 EQ:87 SF:1.5
                                                                                                         000198 EQ:93 SF:1.5
                                                                                                         000200 EQ:93 SF:1.5
                                                                                                         001255 EQ:472 SF:1.5
                      Sa [g]




                                1                                                                        004679 EQ:1635 SF:1.5
                                                                                                         006335 EQ:2142 SF:1.5

                               0.5



                                0
                                     0   0.2     0.4       0.6     0.8          1          1.2     1.4       1.6     1.8         2
                                                                           Period [sec]
     Figure 4. Site class A – Zone 3. Set with minimum single-record deviation from the horizontal and vertical target
                               (δHmax = 1.336; δVmax = 0.799). SF is the individual scale factor.

                                                                         Horizontal SRSS

                                                                                                               000187 EQ:87
                               1.5                                                                             000196 EQ:93
                                                                                                               000197 EQ:93
                                                                                                               000199 EQ:93
                                                                                                               000228 EQ:108
                      Sa [g]




                                1                                                                              006263 EQ:1635
                                                                                                               006334 EQ:2142

                               0.5



                                0
                                     0     0.5         1         1.5            2            2.5         3         3.5           4
                                                                           Period [sec]

                                                                             Vertical

                                                                                                               000187 EQ:87
                               1.5                                                                             000196 EQ:93
                                                                                                               000197 EQ:93
                                                                                                               000199 EQ:93
                                                                                                               000228 EQ:108
                      Sa [g]




                                1                                                                              006263 EQ:1635
                                                                                                               006334 EQ:2142

                               0.5



                                0
                                     0   0.2     0.4       0.6     0.8          1          1.2     1.4       1.6     1.8         2
                                                                           Period [sec]
Figure 5. Site class B – Zone 1. Set with minimum average deviation from the horizontal and vertical target spectrum
                                              (δH = 0.221; δV = 0.171).
                                             1st US-Italy Seismic Bridge Workshop                                                11


                                                                      Horizontal SRSS

                                                                                                            000197 EQ:93
                           1.5                                                                              000199 EQ:93
                                                                                                            000228 EQ:108
                                                                                                            000231 EQ:108
                                                                                                            004673 EQ:1635
                  Sa [g]




                            1                                                                               006263 EQ:1635
                                                                                                            006334 EQ:2142

                           0.5



                            0
                                 0     0.5          1         1.5            2            2.5         3         3.5          4
                                                                        Period [sec]

                                                                          Vertical

                                                                                                            000197 EQ:93
                           1.5                                                                              000199 EQ:93
                                                                                                            000228 EQ:108
                                                                                                            000231 EQ:108
                                                                                                            004673 EQ:1635
                  Sa [g]




                            1                                                                               006263 EQ:1635
                                                                                                            006334 EQ:2142

                           0.5



                            0
                                 0   0.2      0.4       0.6     0.8          1          1.2     1.4       1.6     1.8        2
                                                                        Period [sec]
 Figure 6. Site class B – Zone 2. Set with minimum average deviation from vertical target spectrum (δV = 0.175).

                                                                      Horizontal SRSS

                                                                                                            000181 EQ:87
                           1.5                                                                              000197 EQ:93
                                                                                                            000230 EQ:108
                                                                                                            000536 EQ:250
                                                                                                            002015 EQ:169
                  Sa [g]




                            1                                                                               004673 EQ:1635
                                                                                                            006328 EQ:2142

                           0.5



                            0
                                 0     0.5          1         1.5            2            2.5         3         3.5          4
                                                                        Period [sec]

                                                                          Vertical

                                                                                                            000181 EQ:87
                           1.5                                                                              000197 EQ:93
                                                                                                            000230 EQ:108
                                                                                                            000536 EQ:250
                                                                                                            002015 EQ:169
                  Sa [g]




                            1                                                                               004673 EQ:1635
                                                                                                            006328 EQ:2142

                           0.5



                            0
                                 0   0.2      0.4       0.6     0.8          1          1.2     1.4       1.6     1.8        2
                                                                        Period [sec]
Figure 7. Site class B – Zone 3. Set with minimum average deviation from the vertical target spectrum (δV = 0.338)
                                 with records belonging from seven different events.
12          Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



                                                                       Horizontal SRSS

                                                                                                        000042 EQ:30 SF:1.8
                             1.5                                                                        000879 EQ:349 SF:1
                                                                                                        007329 EQ:2343 SF:1.4
                                                                                                        001226 EQ:472 SF:1.5
                                                                                                        001257 EQ:472 SF:1.3
                    Sa [g]




                              1                                                                         001560 EQ:497 SF:1
                                                                                                        001703 EQ:497 SF:1

                             0.5



                              0
                                   0     0.5         1         1.5            2            2.5          3         3.5           4
                                                                         Period [sec]

                                                                           Vertical

                                                                                                       000042 EQ:30 SF:1.85
                             1.5                                                                       000879 EQ:349 SF:1.85
                                                                                                       007329 EQ:2343 SF:1.85
                                                                                                       001226 EQ:472 SF:1.85
                                                                                                       001257 EQ:472 SF:1.85
                    Sa [g]




                              1                                                                        001560 EQ:497 SF:1.85
                                                                                                       001703 EQ:497 SF:1.85

                             0.5



                              0
                                   0   0.2     0.4       0.6     0.8          1          1.2     1.4        1.6     1.8         2
                                                                         Period [sec]


Figure 8. Site class C – Zone 1. Set with minimum single-record deviation from the horizontal target spectrum (δHmax =
0.525) and minimum average deviation from the vertical target spectrum (δV = 0.125). SF is the individual scale factor.

                                                                       Horizontal SRSS

                                                                                                              000879 EQ:349
                             1.5                                                                              007329 EQ:2343
                                                                                                              001226 EQ:472
                                                                                                              001257 EQ:472
                                                                                                              006959 EQ:473
                    Sa [g]




                              1                                                                               001560 EQ:497
                                                                                                              001703 EQ:497

                             0.5



                              0
                                   0     0.5         1         1.5            2            2.5          3         3.5           4
                                                                         Period [sec]

                                                                           Vertical

                                                                                                        000879 EQ:349 SF:1.75
                             1.5                                                                        007329 EQ:2343 SF:1
                                                                                                        001226 EQ:472 SF:1
                                                                                                        001257 EQ:472 SF:1
                                                                                                        006959 EQ:473 SF:1.7
                    Sa [g]




                              1                                                                         001560 EQ:497 SF:1.8
                                                                                                        001703 EQ:497 SF:1

                             0.5



                              0
                                   0   0.2     0.4       0.6     0.8          1          1.2     1.4        1.6     1.8         2
                                                                         Period [sec]

             Figure 9. Site class C – Zone 2. Unique un-scaled set found. SF is the individual scale factor.
                                              1st US-Italy Seismic Bridge Workshop                                                  13


                                                                       Horizontal SRSS

                                                                                                             000151 EQ:65
                            1.5                                                                              000479 EQ:230
                                                                                                             001726 EQ:561
                                                                                                             007329 EQ:2343
                                                                                                             001226 EQ:472
                   Sa [g]




                             1                                                                               001230 EQ:472
                                                                                                             006959 EQ:473

                            0.5



                             0
                                  0     0.5          1         1.5            2            2.5         3         3.5           4
                                                                         Period [sec]

                                                                           Vertical

                                                                                                       000151 EQ:65 SF:1.75
                            1.5                                                                        000479 EQ:230 SF:2
                                                                                                       001726 EQ:561 SF:1.3
                                                                                                       007329 EQ:2343 SF:1
                                                                                                       001226 EQ:472 SF:1
                   Sa [g]




                             1                                                                         001230 EQ:472 SF:1.8
                                                                                                       006959 EQ:473 SF:1.45

                            0.5



                             0
                                  0   0.2      0.4       0.6     0.8          1          1.2     1.4       1.6     1.8         2
                                                                         Period [sec]

  Figure 10. Site class C – Zone 3. Set with minimum single-record deviation from the horizontal and vertical target
  spectrum (δHmax = 1.274; δVmax = 1.163) and minimum average deviation from the horizontal target spectrum (δH =
                                        0.147). SF is the individual scale factor.

4.2 Normalized (non-dimensional) sets
The input database has also been examined for records normalized, which should allow
to select records having a spectral shape similar to that of the code. However, this entails
scaling the records, which was avoided, if possible, in the analyses presented in the
previous sections. Here, the records have been rendered non-dimensional by dividing the
spectral ordinates by their spectral acceleration at T = 0.04 sec (assumed to be an
approximation of the peak ground acceleration). After normalizing the spectra, the same
analyses discussed above have been repeated. In fact, combinations of these spectra have
been compared to the non-dimensional code spectra, and then the results (summarized in
Table 4) may be considered country-independent because they’re independent of the ag
values. From Figure 11 to Figure 13 some results for the horizontal SRSS components
are given for A, B and C site classes. As expected normalization of spectra reduces the
spectral variability within a set while keeping a good average matching with the code if
large scale factors (given in Table 5) are allowed.

                Table 4. Number of normalized (non-dimensional) compliant sets found
        Ground Compatibility Lower Bound7                            Compatibility Upper Bound6                        Sets Found
        A      10%                                                   100%                                              9053
        B      10%                                                   100%                                              53465
        C      10%                                                   100%                                              8139
14           Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



                                                         Horizontal SRSS;Non−dimensional

                                 4                                                              000172 EQ:81
                                                                                                000286 EQ:146
                                                                                                001255 EQ:472
                                 3                                                              002553 EQ:239
                     Sa [adim]




                                                                                                004674 EQ:1635
                                                                                                005821 EQ:1888
                                 2                                                              007115 EQ:2302


                                 1


                                 0
                                     0      0.5      1     1.5          2         2.5      3          3.5        4
                                                                   Period [sec]


 Figure 11 – Site class A. Set with minimum average deviation from the horizontal target spectrum (δH = 0.092) with
                                    records belonging from seven different events.

                                                         Horizontal SRSS;Non−dimensional

                                 4                                                              000232 EQ:108
                                                                                                000500 EQ:239
                                                                                                002019 EQ:709
                                 3                                                              002030 EQ:169
                     Sa [adim]




                                                                                                005809 EQ:1887
                                                                                                006142 EQ:559
                                 2                                                              007143 EQ:2309


                                 1


                                 0
                                     0      0.5      1     1.5          2         2.5      3          3.5        4
                                                                   Period [sec]


 Figure 12 – Site class B. Set with minimum average deviation from the horizontal target spectrum (δH = 0.066) with
                                    records belonging from seven different events.

                                                         Horizontal SRSS;Non−dimensional

                                                                                                 005673 EQ:83
                                 4
                                                                                                 000175 EQ:83
                                                                                                 000555 EQ:264
                                 3                                                               001251 EQ:472
                     Sa [adim]




                                                                                                 006961 EQ:473
                                                                                                 006963 EQ:473
                                 2                                                               006967 EQ:473


                                 1


                                 0
                                     0      0.5      1     1.5          2         2.5      3          3.5        4
                                                                   Period [sec]




     Figure 13 – Site class C. Set with minimum average deviation from the horizontal target spectrum (δH = 0.079).


          Table 5. Average scaling factor to match the code spectra of non-dimensional records
                                         Site/Zone                 ag                           SF
                                             A-1                  0.35                         11.6
                                             A-2                  0.25                          8.3
                                             A-3                  0.15                          5.0
                                             B-1                  0.35                         24.2
                                             B-2                  0.25                         17.3
                                             B-3                  0.15                         10.4
                                1st US-Italy Seismic Bridge Workshop                              15


                          C-1                  0.35                    10.8
                          C-2                  0.25                    7.7
                          C-3                  0.15                    4.6



5. CONCLUSIONS
In the presented study the EC8 Part 2 prescriptions for selection of the seismic input for
non-linear time-history analysis of bridges have been reviewed. The code requires the use
of real records consistent with magnitude, distance and faulting mechanism driving the
seismic hazard. The chosen record set should have the average of the SRSS larger than
1.3 times the code spectrum, while the average of the vertical components (to be
considered in special cases only) should be not lower than 0.9 times the vertical code
spectrum in a certain range of periods. The European Strong-motion database was
investigated to search for record sets matching the EC8 Part 2 spectral requirements. The
analyses performed did not try to match any specific magnitude, distance and mechanism
because this study do not refer to any site-specific application, however it has to be noted
that the code spectrum is only indirectly related to seismic hazard at the site, and hazard
disaggregation parameters are not always available to engineers. The exercise performed
allowed to find results, some of which are presented herein. The sets are of two types: (1)
combinations made of records which do not require scaling to match the code spectrum
compatibility requirement; (2) records to be scaled to comply with the code spectral
prescription, which reduces the variability within a set at the price of eventually high
scaling factors.

ACKNOWLEDGEMENTS
The study presented in this paper was developed within the activities of Rete dei
Laboratori Universitari di Ingegneria Sismica – ReLUIS for the research program
founded by Dipartimento di Protezione Civile.

REFERENCES
Ambraseys, N., Smit, P., Berardi, R., Rinaldis, D., Cotton, F. and Berge, C. [2000], Dissemination of
   European Strong-Motion Data (CD-ROM collection). European Commission, DGXII, Science,
   Research and Development, Bruxelles.
Ambraseys, N.N., Douglas, J., Rinaldis, D., Berge-Thierry, C., Suhadolc, P., Costa, G.,
   Sigbjornsson, R. and Smit, P. [2004], Dissemination of European strong-motion data, Vol. 2, CD-
   ROM Collection, Engineering and Physical Sciences Research Council, United Kingdom.
  16          Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



  Beyer, K., Bommer, J.J. [2007], “Selection and scaling of real accelerograms for bi-directional
       loading: a review of current practice and code provisions,” Journal of Earthquake Engineering,
       11(Supplement 1), 13-45.
  Cornell, C.A. [2004] “Hazard, Ground motions and Probabilistic assessment for PBSD,” In
       Performance based seismic design concepts and implementation. PEER Report 2004/05. Pacific
       Earthquake Engineering Research Center University of California Berkeley, CA, USA.
  CEN [2003] Eurocode 8: Design of structures for earthquake resistance. Part 1: General rules, seismic actions
       and rules for buildings, Final Draft prEN 1998, European Committee for Standardization,
       Brussels, Belgium.
  CEN [2005] Eurocode 8: Design of structures for earthquake resistance. Part 2: Bridges”, Final Draft prEN
       1998-2, European Committee for Standardization, Brussels, Belgium.
  Iervolino, I., Maddaloni G., Cosenza E. [2007], “Eurocode 8 compliant real record sets for seismic
       analysis of structures,” Journal of Earthquake Engineering, in press.
  Naeim F., Alimoradi A., Pezeshk S. [2004] “Selection and scaling of ground motion time histories
       for structural design using genetic algorithms,” Earthquake Spectra, 20(2): 413–426.
  Ordinanza del Presidente del Consiglio dei Ministri (OPCM) n. 3519 [2006] “Criteri per
       l’individuazione delle zone sismiche e la formazione e l’aggiornamento degli elenchi delle
       medesime zone,” Gazzetta Ufficiale della Repubblica Italiana, 108.


  APPENDIX
  In this appendix the details about the records presented in the paper are given as they
  come from the European Strong-motion Database website.

       Table 6. A-type site class: record information for the un-scaled type sets (Figure 2 to Figure 4)

Site/Zone Code            Event Name            Country           Date                Station Name
   A-1    000055             Friuli               Italy        06/05/1976         Tolmezzo-Diga Ambiesta
          000182             Tabas                Iran         16/09/1978                Dayhook
                                                Serbia &
            000198        Montenegro                           15/04/1979           Ulcinj-Hotel Albatros
                                               Montenegro
            000290      Campano Lucano            Italy        23/11/1980                   Sturno
                         South Iceland
            006332                               Iceland       21/06/2000                Thjorsartun
                          (aftershock)
                         South Iceland
            006349                               Iceland       21/06/2000                Thjorsarbru
                          (aftershock)
            007142           Bingol              Turkey        01/05/2003        Bingol-Bayindirlik Murlugu
  A-2       000055            Friuli              Italy        06/05/1976        Tolmezzo-Diga Ambiesta
            000182            Tabas               Iran         16/09/1978                Dayhook

            000198        Montenegro             Serbia &      15/04/1979           Ulcinj-Hotel Albatros
                                  1st US-Italy Seismic Bridge Workshop                                    17


                                             Montenegro
                                              Serbia &                         Hercegnovi Novi-O.S.D.
          000200         Montenegro                     15/04/1979
                                             Montenegro                             Pavicic School
                                                                               Gebze-Tubitak Marmara
          001228             Izmit              Turkey       17/08/1999
                                                                                  Arastirma Merkezi
          004674        South Iceland          Iceland       17/06/2000            Flagbjarnarholt
          007142           Bingol              Turkey        01/05/2003       Bingol-Bayindirlik Murlugu
  A-3     000052            Friuli               Italy       06/05/1976                 Feltre
          000182           Tabas                 Iran        16/09/1978               Dayhook
                                              Serbia &
          000198         Montenegro                          15/04/1979          Ulcinj-Hotel Albatros
                                             Montenegro
                                              Serbia &                        Hercegnovi Novi-O.S.D.
          000200         Montenegro                          15/04/1979
                                             Montenegro                            Pavicic School
          001255             Izmit             Turkey        17/08/1999        Heybeliada-Senatoryum
          004679        South Iceland          Iceland       17/06/2000      Hveragerdi-Retirement House
                        South Iceland
          006335                               Iceland       21/06/2000             Selfoss-City Hall
                         (aftershock)



      Table 7. B-type site class: record information for the un-scaled type sets (Figure 5 to Figure 7)

Site/Zone Code         Event Name                   Country        Date               Station Name
   B-1    000187           Tabas                      Iran      16/09/1978                 Tabas
          000196        Montenegro                 Yugoslavia   15/04/1979         Petrovac-Hotel Oliva
          000197        Montenegro                 Yugoslavia   15/04/1979         Ulcinj-Hotel Olimpic
          000199        Montenegro                 Yugoslavia   15/04/1979         Bar-Skupstina Opstine
          000228 Montenegro (aftershock)           Yugoslavia   24/05/1979         Bar-Skupstina Opstine
          006263       South Iceland                Iceland     17/06/2000               Kaldarholt
          006334 South Iceland (aftershock)         Iceland     21/06/2000               Solheimar
   B-2    000197        Montenegro                 Yugoslavia   15/04/1979         Ulcinj-Hotel Olimpic
          000199        Montenegro                 Yugoslavia   15/04/1979         Bar-Skupstina Opstine
          000228 Montenegro (aftershock)           Yugoslavia   24/05/1979         Bar-Skupstina Opstine
          000231 Montenegro (aftershock)           Yugoslavia   24/05/1979            Tivat-Aerodrom
          004673       South Iceland                Iceland     17/06/2000                 Hella
          006263       South Iceland                Iceland     17/06/2000               Kaldarholt
          006334 South Iceland (aftershock)         Iceland     21/06/2000               Solheimar
   B-3    000181           Tabas                      Iran      16/09/1978               Boshroyeh
          000197        Montenegro                 Yugoslavia   15/04/1979         Ulcinj-Hotel Olimpic
  18          Iunio Iervolino, Giuseppe Maddaloni, Edoardo Cosenza and Gaetano Manfredi



              000230     Montenegro (aftershock) Yugoslavia 24/05/1979                 Budva-PTT
                                                                                     Tercan-Meteoroji
              000536          Erzincan                Turkey      13/03/1992            Mudurlugu
              002015   Kefallinia (aftershock)        Greece      23/03/1983      Argostoli-OTE Building
              004673       South Iceland              Iceland     17/06/2000               Hella
              006328 South Iceland (aftershock)       Iceland     21/06/2000            Kaldarholt


       Table 8. C-type site class: record information for the un-scaled type sets (Figure 8 to Figure 10)

Site/Zone Code              Event Name         Country       Date                  Station Name
   C-1    000042               Ionian          Greece     04/11/1973           Lefkada-OTE Building
          000879               Dinar            Turkey    01/10/1995        Dinar-Meteoroloji Mudurlugu
          007329                Faial          Portugal   09/07/1998                    Horta
          001226                Izmit           Turkey    17/08/1999        Duzce-Meteoroloji Mudurlugu
          001257                Izmit           Turkey    17/08/1999              Yarimca-Petkim
                                                                              Bolu-Bayindirlik ve Iskan
              001560          Duzce 1        Turkey 12/11/1999                       Mudurlugu
              001703          Duzce 1        Turkey 12/11/1999              Duzce-Meteoroloji Mudurlugu
  C-2         000879            Dinar        Turkey 01/10/1995              Dinar-Meteoroloji Mudurlugu
              007329             Faial       Portugal 09/07/1998                        Horta
              001226            Izmit        Turkey 17/08/1999              Duzce-Meteoroloji Mudurlugu
              001257            Izmit        Turkey 17/08/1999                    Yarimca-Petkim
              006959      Izmit (aftershock) Turkey 13/09/1999              Adapazari Bahtiyat Topcu Evi
                                                                              Bolu-Bayindirlik ve Iskan
              001560           Duzce 1        Turkey 12/11/1999                      Mudurlugu
              001703           Duzce 1        Turkey 12/11/1999             Duzce-Meteoroloji Mudurlugu
  C-3         000151      Friuli (aftershock)  Italy 15/09/1976                         Buia
              000479             Manjil         Iran   20/06/1990                      Rudsar
              001726             Adana        Turkey 27/06/1998             Ceyhan-Tarim Ilce Mudurlugu
              007329              Faial       Portugal 09/07/1998                       Horta
              001226             Izmit        Turkey 17/08/1999             Duzce-Meteoroloji Mudurlugu
              001230             Izmit        Turkey 17/08/1999           Iznik-Karayollari Sefligi Muracaati
              006959      Izmit (aftershock) Turkey 13/09/1999              Adapazari Bahtiyat Topcu Evi
                                  1st US-Italy Seismic Bridge Workshop                             19




          Table 9. Record information for the non-dimensional type sets (Figure 11 to Figure 13)

Site Code           Event Name               Country        Date                Station Name
 A 000172           Basso Tirreno              Italy     15/04/1978                Messina 1
     000286       Campano Lucano               Italy     23/11/1980                 Arienzo
     001255             Izmit                 Turkey     17/08/1999         Heybeliada-Senatoryum
     002553             Racha                Georgia     29/04/1991                  Toros
     004674         South Iceland            Iceland     17/06/2000             Flagbjarnarholt
     005821    Strofades (aftershock)        Greece      18/11/1997        Koroni-Town Hall (Library)
     007115           Pulumur                 Turkey     27/01/2003        Bingol-Bayindirlik Murlugu
 B 000232     Montenegro (aftershock)       Yugoslavia   24/05/1979           Kotor-Naselje Rakite
     000500             Racha                Georgia     29/04/1991               Bogdanovka
              Off coast of Magion Oros
    002019            peninsula               Greece     06/08/1983          Ierissos-Police Station
    002030     Kefallinia (aftershock)        Greece     23/03/1983        Zakynthos-OTE Building
    005809            Strofades               Greece     18/11/1997        Argostoli-OTE Building
    006142             Aigion                 Greece     15/06/1995       Patra-San Dimitrios Church
                                                                          Elazig-Bayindirlik ve Iskan
   007143               Bingol                Turkey     01/05/2003                Mudurlugu
 C 005673                Volvi                Greece     20/06/1978      Pehcevo-Duvanski Kombinat
   000175                Volvi                Greece     20/06/1978         Thessaloniki-City Hotel
   000555              Kallithea              Greece     18/03/1993           Patra-OTE Building
   001251                Izmit                Turkey     17/08/1999           Bursa-Tofa Fabrikasi
   006961          Izmit (aftershock)         Turkey     13/09/1999       Adapazari A.Babalioglu Evi
   006963          Izmit (aftershock)         Turkey     13/09/1999    Akyazi Gebes Koyu Imam Lojmani
   006967          Izmit (aftershock)         Turkey     13/09/1999        Ambarli-Termik Santrali

								
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